A substantial portion of hydrocarbon fractions used to prepare fuels such as motor gasoline are derived from catalytic cracking of heavy petroleum fractions such as vacuum gas oils. The cracked materials are typically distilled into fractions including naphtha, heating oil, and diesel fuel.
Cracked naphthas are obtained from catalytic crackers in relatively high volumes and generally have good octane numbers. They are major components in blending for the motor gasoline pool. A substantial portion of the octane rating of cracked naphtha is due to its high olefin content.
Among the primary contaminants in feeds to fluidized catalytic cracking units are sulfur bearing materials. These sulfur contaminants result in the appearance of sulfur species in the cracked products. Heavy gasoline components are known to accumulate sulfur impurities.
Combustion of gasoline and diesel fuels during use in internal combustion engines converts the sulfur contaminants into sulfur oxides. The sulfur oxides are environmentally undesirable and also have been found to have a long-term deactivation impact on automotive catalytic converters that are used to remove nitrogen oxide and unburned hydrocarbon contaminants from automotive emissions. In order to improve air quality, environmental protection agencies of various industrialized countries have therefore announced or proposed new regulations requiring reduction in sulfur content of gasoline, diesel, and other motor fuels.